We have multiple users that are assigned MacBook Pro's who we know never take them away from their desks. We need some sort of proof to repurpose this hardware and issue them an iMac. We would really like a terminal command we can send or a script we could run to determine the last time the laptop was unplugged.

So far everything we've found just tells you its current state, but not "timer"?

pmset -g pslog will log the state of system's power. As the device is unplugged or plugged you'll see the log update. This could show how frequently a notebook is unplugged and charged. If it remains plugged in for long spans of time, or is often drawing from 'AC Power' then you'd know if it's mobile or not.

I'll third the suggestion of pmset's logs, however an easier way to access that log (tested under 10.8) is to open System Information, go down to the Software section, pick the Logs subsection and you should see "Power Management logs". Hope this helps!